Cross flow evaporators typically are mounted vertically or nearly so with parallel pairs of refrigerant flow tubes extending between substantially horizontal, upper and lower manifolds. Especially in evaporators of compact design and high capacity, the refrigerant flow tubes are closely spaced, and the lower manifold is significantly wider than the edge to edge width of the flow tubes. Ambient air with substantial relative humidity is blown across the refrigerant flow tubes, condensing thereon and draining down toward the lower manifold. Because of the close spacing of the tubes and width of the lower manifold, condensed water tends to build up in columns between the lower ends of the tubes, blocked by the lower manifold These columns rise to and dynamically maintaining a characteristic height dependent on the dimensions of the particular core in question and the humidity, forming a slightly concave meniscus film that bulges out minutely past the front and back edges of the closely spaced pairs of tube ends. These retained columns of water can block air flow sufficiently to affect the efficiency of the core.
One known and straightforward response has been to purposely stamp individual drain troughs or grooves directly into the surface of the lower manifold, between the pairs of tube ends. A typical example may be seen in U.S. Pat. No. 7,635,019, and there are numerous variations of the same basic theme. This requires dedicated dies and tools for the lower manifold, of course, and can disrupt the flow of refrigerant in the lower manifold.